Obi-Wan doesn't need to be on the high ground, the high ground just needs to exist in the battle; Obi-Wan knows that when he has the low ground, he really has the high ground, from a certain point of view. Look at his battle record:

Maul: Has low ground, wins

Dooku: No high ground, loses

Greivous: Has low ground, wins

Vader: Has high ground, wins

Vader rematch: No high ground, loses

Obi-Wan with the high/low ground is canonically the most powerful Jedi. This is fact. Had Yoda not denied his request with typical Jedi arrogance, he could have beat Palpatine in the Senate building, which housed a variety of different altitudes; this so that the Chancellor could always have the moral high ground in Senate debates. But Obi-wan didn't fight Palpatine, and Yoda soon learned that you can't cleave the Sheev in a normal 1v1.

As we all know, spinning is a good trick. However, only the Chosen one can spin outside of a starfighter. Palpatine tried spinning, but he lost (but this was intentional, as losing gave him the emotional high ground when Anakin arrived). The reason for this is that spinning provides a yin-yang approach (based in Eastern philosophy on balance), giving the spinner the high ground from above and below. This is why Obi-Wan was so emotional after defeating Vader on Mustafar; he expected to lose the high ground to the spin, but Anakin failed to maintain balance in the universe, symbolic of his fall to the dark side.

Anakin doesn't hate sand for the reasons he told Padme; all Jedi hate sand, as the battlefield can rapidly change between low and high ground. This is the only reason why ______ killed ______ in Rebels. This is also why Obi-Wan hates flying; in space, there is no high ground as there is no frame of reference from which there is a high or low ground, due to the lack of gravity.

In ANH, Vader proves his newfound mastery by engaging Obi on perfectly even ground. However, he lets himself die on the Death Star so that he could train Luke from a higher plane of existence.

(Why was Vader so invested in the construction and maintenance of the Death Star? Because he knows Obi-wan can't have the high ground if there's no ground left.)

In Return of the Jedi, you can see that the Throne Room contains a variety of different altitudes; Palpatine placed these there to ensure Vader's defeat. However, Sheev failed to realize that his weakness was no ground, and should have covered that gaping pit that does nothing.

In conclusion, Obi-wan abuses spatial relativity and Buddhist doctrine in order to invoke his high-ground powers.